Let's think more globally.
Bookmarking is necessary not only for Audio Books. Currently I have a [Cue] list-field, where I put Timecode with Description on each part of the music. I use it for my video-editing purposes. Also, bookmarks are necessarly needed to be added for files which contain multiple tracks, for DJ Sets, long Mix-tracks, etc.
All of the above can be typed directly to some [Custom Field].
But, it would be lovely to see my [Cue] field Alive. I think additional Field Data Type should be added: "List (playlist)" or "List (cuelist)" with additional window to edit it's values, which will consist from two columns: [Timecode] [Description]. The difference from common "List (semicolon delimited)" Data Type would be that after selecting an item from the list, MC will automatically play the music starting from the timecode contained in the selected line.
A Factory Default [Lyrics Synchronized] field can be added to demonstrate the power of that kind of Field. An example of what data would this field contain:
[00:00:40] Talk to me
[00:00:44] Want my love
[00:00:48] Talk to me
[00:00:52] But not that stuff
[00:00:59] You ain't enough
[00:01:07] Usin' my love
[00:01:11] 'n' you call for me
[00:01:14] And again I see
[00:01:19] In the heat of the night
[00:01:21] You loose your heart
[00:01:23] And sell your soul
[00:01:25] I loose control
[00:01:27] In the heat of the night
I will extend the idea of possible functionality of this field. Additional Factory Default Field of this type should be added, into which MC will write automatically a Timecode at which the file was stopped or when a user switched to another track. In case if a file was listened to the end, this Field would contain [00:00:00]